The quenching process is a crucial step in
forging, designed to alter the microstructure of metal through rapid cooling, thereby enhancing the hardness, strength, and wear resistance of the forged components. Since different shapes, materials, and sizes of
forgings have varied requirements, selecting the appropriate quenching method is essential. There are up to ten commonly used quenching methods, each tailored for specific forging applications and process needs. Below is a detailed explanation of these common quenching techniques used in forging.
Single-medium quenching is the most basic method, where the heated forging is quickly immersed in a single medium (such as water, oil, or air) for cooling. This causes the internal structure of the forging to change. While simple, this method requires strict control over the material, shape, and size of the forging.
Applications: Suitable for simpler shapes like shafts, gears, and bolts, which achieve uniform hardening and improved mechanical properties.
Medium Options: Water (fast cooling, for forgings needing rapid quenching), oil (slower cooling, for forgings resistant to deformation), or air (for forgings with lower cooling speed requirements or strict deformation control).
In dual-medium quenching, the forging is first immersed in a fast-cooling medium (such as water) until it approaches the martensitic start temperature (Ms), then transferred to a slower-cooling medium (such as oil or air) to cool to room temperature. This method controls cooling speed, reducing deformation and cracking risks.
Applications: Ideal for complex-shaped or large forgings, especially high-carbon or alloy steels, such as large molds or engine parts. It reduces stress concentration, ensuring mechanical performance.
Medium Combinations: Commonly used combinations include water-oil or water-air, which provide a balance between surface hardness and internal toughness for stability in complex conditions.
Martensitic step quenching involves immersing the forging into a liquid medium (like a salt or alkali bath) just above or below the Ms point, holding it for a period, and then air cooling. This prevents internal stresses and deformation caused by rapid cooling.
Applications: Best for small, high-strength, complex-shaped forgings, especially precision parts like tools, molds, and bearings.
Advantages: Reduces deformation and cracking, making it suitable for forgings requiring high dimensional accuracy.
This method cools the forging in a medium below the Ms point but above the Mf point, allowing for a fast cooling rate without excessive internal stress. This reduces the chances of quenching cracks.
Applications: Suitable for larger or low-hardenability steel forgings, such as heavy machinery parts and structural components, ensuring uniform internal structure with minimal distortion.
Bainitic isothermal quenching rapidly cools the forging below the bainite transformation temperature and holds it for an isothermal process, allowing it to transform into a bainite structure. This improves wear resistance and toughness.
Applications: Common for small alloy steel and high-carbon steel parts like gears and bolts that require high wear resistance.
Process: Involves austenitization, rapid cooling, and isothermal treatment to achieve optimal bainitic structure, enhancing overall performance.
Combined quenching incorporates both martensitic and bainitic isothermal quenching, cooling the forging rapidly below the Ms point to form martensite, followed by an isothermal treatment in the lower bainite region. This method combines the hardness of martensite with the toughness of bainite.
Applications: Used for large tool steel forgings or components facing complex conditions, such as large gears and heavy equipment parts, ensuring both hardness and toughness for extreme conditions.
Also known as step-up isothermal quenching, this method pre-cools the forging in a medium below the Ms point, then transfers it to a high-temperature bath for isothermal treatment. It precisely controls the cooling rate, preventing uneven deformation caused by fast cooling.
Applications: Mainly used for steel forgings with poor hardenability or large-sized forgings requiring strict deformation control, such as large structural parts or machinery bases.
In delayed quenching, the forging is pre-cooled in air, warm water, or a salt bath to just above the Ar3 or Ar1 temperature before transferring to a single quenching medium for rapid cooling. This reduces temperature gradients and minimizes deformation risks.
Applications: Suitable for complex-shaped or uneven-thickness forgings, like precision mechanical parts or large forgings.
Self-tempering quenching involves rapidly cooling only the working portion of the forging, while the non-cooled portion self-tempers the surface through heat conduction. This results in surface hardness combined with tempered toughness.
Applications: Primarily used for tool forgings like hammers and chisels, which require high surface hardness and sufficient toughness to handle impact loads.
Jet quenching uses jets of cooling medium (usually water) to cool the forging. The intensity and speed of the water can be adjusted to ensure even hardening depth and hardness across the surface and interior of the forging.
Applications: Used for localized quenching of large gears and shafts, ideal for mechanical parts needing high surface hardness.
Quenching is a vital process in forging production, and different methods impact forging performance in various ways. By selecting the appropriate quenching technique, forgings can achieve the desired hardness while maintaining strength and toughness, minimizing deformation and cracking risks. Each type of forging requires a carefully chosen quenching method to ensure optimal performance during use.
